Do alleles automatically become more dominant if selection is favoring them?

Recall that there are three things that change in allele frequencies in a population... So yes, that is roughly one of the reasons.

1. Selection. This is where traits of individuals allow some to reproduce more offspring than others. This can be 'natural selection', where these are traits that give some advantage in the environment they are living in; 'artificial selection', where humans are specifically choosing which individuals get to breed; 'sexual selection', where members of one sex choose certain traits in members of the opposite sex, etc.

2. Genetic drift. This is where alleles propagate through random matings or events. For example, a recessive or neutral allele (which has no effect in individuals) can go up or down in frequency just through random matings; or a disease or natural disaster could wipe out half the population and by chance wipe out a part of a population where more blue-eyed individuals happen to be living ... which decreases the allele frequency of blue eyes, but not because blue eyes makes one more vulnerable to diseases or natural disasters.

3. Gene flow. This is where individuals move between populations, carrying their genes with them. For example, an allele X may gain some frequency in population A, and if some individual from that population travels to population B, then allele X may start propagating in that population as well.

Next Q...

Because inheritance of a harmful dominant allele can be lethal, these alleles tend to be quite rare in the population. When there are harmful alleles in a population, species of that population tend to stay away (prefer not to mate) from them as they are unfavourable for survival. These individuals are not fit for survival and so doesn't increase the overall fitness of the species.

There are traits that are not favourable, but are still dominant when expressed.

The most harmful genes are recessive because that is really the way they can survive. If a harmful gene were dominate, the individual likely won't live long enough to have offspring (although modern medicine is changing this situation). That's why dominate harmful genes are so rare.

(Look up how Sickle Cell Aenima has survived if you want want a really cool harmful genetic disorder that has a very big advantage to some of those who carry it).

Recessive genes are pretty cool because they are able to hide within a population, without falling out of a gene pool. (It's important to keep in mind that recessive genes are as likely to fall out of a gene pool as dominate genes). For instance - my dad has blue eyes, and my mom has brown eyes. I have brown eyes but I am carrying the blue eye gene - but it doesn't show up. I am as likely to pass on my dominate brown eyed gene as my blue eyed gene - thus the survival of either is 50/50. The same goes for harmful genes. They are able to survive in the gene pool and are able to be passed on because they, like my dad's blue eyed gene, are able to hide. Unless I marry someone who also has a blue eyed gene (ha! - if only I can find a sucker) then there is no way my blue eyed gene could show up. Harmful recessive genes work the same way.

It's for this reason that testing oneself of genetic disorders before having babies isn't a bad idea, especially if harmful genetic diseases have shown up in your family. Two individuals could be carring a very nasty recessive gene and have NO CLUE it's in the genetic make-up - but could easily show up in their children. Good mechanism for hinding and survival, don't you think?